1. Field of the Invention
The present invention relates to an electrode structure for controlling an alignment of liquid crystal molecules in an OCB (which is an abbreviation for optically compensated birefringence and hereinafter abbreviated to “OCB”) mode liquid crystal display device.
2. Description of the Related Art
A TN (twisted nematic) mode which is widely used at present in a liquid crystal display device has a high contrast, but has a problem in that visual dependence is remarkable. Thus, in order to solve such a problem, various characteristic improving methods have been proposed, centering on a pixel dividing method in which pixels are divided into regions and liquid crystal molecules are controlled in each of the divided regions. However, a liquid crystal display device using a nematic liquid crystal generally has a low response speed. That is, a response time required for changing gray scale display reaches about 100 ms in maximum. Thus, it cannot be adapted to the display of a moving image for which a high response speed is required. Therefore, a display mode enabling a wide viewing angle and a high speed response, and suitably used in a moving image LCD (liquid crystal display), has been required.
The OCB mode LCD is described as an LCD having a high speed response in addition to a wide viewing angle (see Y. Yamaguchi, et al., SID'93, Digest, pp. 277-280 or JP 07-084254 A). A liquid crystal cell used in the OCB mode is made to stay in a bend alignment state, and is also called a n cell. JP 55-142316 also indicates that the n cell indicates a high speed response.
FIG. 1 shows an example of a fundamental configuration of the OCB mode. A liquid crystal layer 915 in a bend alignment state, sandwiched between two glass substrates 901 and 921, which are disposed to overlap each other so that rubbing directions thereof are parallel to each other, is sandwiched by negative birefringence compensation plates 956 and 966. The negative birefringence compensation plates 956 and 966 are made of discotic liquid crystal; are optically negative; and each have a structure in which a tilt of the principal axis in a layer changes. Furthermore, the negative birefringence compensation plates 956 and 966 are sandwiched by two polarizers 916 and 936. In such a structure of a liquid crystal display device, the bend alignment of the liquid crystal layer 915 always exhibits a self-compensation capability in a rubbing direction and an optically symmetric characteristic owing to its structure.
A transition from liquid crystal molecules in a bend alignment state to the same in another alignment state becomes maximum in a plane which is parallel to the direction of an optical axis, that is, an orientation direction of the liquid crystal molecule at interfaces between the liquid crystal layer 915 and the two glass substrates 901 and 921, and perpendicular to the substrates. In addition, in the case where an optical anisotropic medium is sandwiched by two polarizers whose transmission axes are orthogonal to each other, a maximum intensity of transmission light is obtained when the optical axis is made to have an angle of 45 degrees relative to the transmission axis of the polarizer. Therefore, in the case where an liquid crystal cell in a bend alignment state is sandwiched by two polarizers whose transmission axes are orthogonal to each other, change of birefringence in a liquid crystal display device becomes maximum when the optical axis of a liquid crystal molecule in the liquid crystal layer is disposed to have an angle of 45 degrees relative to the transmission axis of the polarizer. In the case where the rubbing direction is fixed to a horizontal direction, a maximum intensity of transmission light in the liquid crystal display device is obtained when the transmission axes of the two polarizers 916 and 936 are disposed to have 45 degrees relative to each other.
A driving method for the OCB-mode liquid crystal display device can be classified into two methods, i. e., a normally black LCD to perform black display at a low voltage and a normally white LCD to perform black display at a high voltage. In the case of the normally black LCD in which birefringence to be compensated for is large, a light leakage due to wavelength dispersion is large resulting in difficulty in obtaining sufficient contrast. Therefore, JP 08-327822 A discloses a technique to solve the above-stated problem by employing two negative birefringence compensation plates shown in FIG. 1 to realize a normally black LCD. In more detail, almost all liquid crystal molecules except for molecules near the interface between a liquid crystal layer and an alignment layer are vertically aligned at a high voltage. When residual birefringences in both interfaces between the liquid crystal layer 915 and the two glass substrates 901, 921 are compensated by the two negative birefringence compensation plates, a wide viewing angle characteristic is obtained.
As described above, the OCB mode having superior characteristics such as a wide viewing angle and a high speed response includes large problems. Liquid crystal molecules of a bend alignment cell used in the OCB mode are aligned in a splay alignment state as an initial alignment state and therefore, when a power source is turned on, the liquid crystal molecules thus aligned in all pixels have to be aligned in a bend mode. In addition, during operation for displaying an image, it is necessary to continuously apply a voltage equal to or larger than a critical voltage Vc, which makes the bend alignment of a liquid crystal cell more stable than the splay alignment thereof, to the liquid crystal cell.
The critical voltage Vc is obtained as follows. That is, change of Gibbs energy versus change of a voltage, which is measured with respect to a liquid crystal cell in a splay alignment state and in a bend alignment state, is calculated based on respective parameters such as a physical property value of a liquid crystal material, a gap between two glass substrates, and a pretilt angle associated with a liquid crystal molecule, and changes of curves associated with both cases of a liquid crystal cell in a splay alignment state and in a bend alignment state are compared with each other. In this case, the alignment exhibiting smaller Gibbs energy is more stable. Gibbs energy versus a voltage applied to the liquid crystal cell is plotted with respect to the splay alignment and the bend alignment of the liquid crystal cell, Gibbs energy being plotted on the ordinate and a voltage being plotted on the abscissa, and then, the applied voltage at the intersection of curves of Gibbs energy, which curves correspond to the splay alignment and the bend alignment, is determined as a critical voltage Vc.
An example of Gibbs energy thus calculated is shown in FIG. 2. Theoretically, when a voltage equal to or larger than the critical voltage Vc is applied to a liquid crystal cell, the bend alignment becomes more stable than the splay alignment. However, in order to quickly change the splay alignment of a liquid crystal cell to the bend alignment thereof, it is necessary to apply a far higher voltage to the liquid crystal cell. When a voltage of about 20 V is applied as a high voltage to a liquid crystal cell, the change is completed within a short time of several seconds or less. However, when a liquid crystal cell is driven by an active matrix element, only about 5 V as a maximum voltage can be applied to the liquid crystal cell in terms of protection of a thin film transistor. Application of voltage of 5 V cannot or rarely can make a transition from a liquid crystal cell in a certain alignment state to the same in a desired alignment state, which phenomenon is verified by the inventor's experiments.
To address such a problem, the following methods are proposed.
According to JP 09-218411 A, the following method is described. That is, a spacer as a gap material capable of horizontally aligning liquid crystal molecules near the surface of the spacer and at the same time, serving as nucleus generating means for making a transition from the molecules in a certain alignment state to the same in a bend alignment state is utilized, enabling liquid crystal molecules to stably stay in a bend alignment state without necessity to change normal steps for manufacturing a liquid crystal display device.
According to JP 10-142638 A, the following method is described. That is, a spacer having a diameter smaller than a gap between two glass substrates and equipped with an ability to align liquid crystal molecules vertically near the surface of the spacer is used, and the liquid crystal molecules just on the spacer are aligned perpendicular to the spacer to obtain liquid crystal molecules in a pseudo-hybrid alignment state, thereby promoting the transition from liquid crystal molecules in a splay alignment state to the same in a bend alignment state.
Also, according to JP 10-020284 A, the following method is described. That is, a convex portion made of a material having a dielectric constant higher than that of a liquid crystal layer or a convex portion made of a conductive material and having a taper is formed on each pixel electrode to partially produce a strong electric field. Or, a region for aligning liquid crystal molecules in a high pretilt angle is provided on each pixel electrode to partially obtain liquid crystal molecules aligned at a high pretilt angle with respect to the surface of a substrate, whereby the region serves as nucleus generating means for making a transition from liquid crystal molecules in a certain alignment state to the same in a bend alignment state.
Also, according to JP 2000-330141 A, the following method is described. That is, a hybrid alignment film consisting of a horizontally aligning component and a vertically aligning component is used to make liquid crystal molecules largely tilted with respect to the surface of a substrate, so that liquid crystal molecules are in a bend alignment state even when no voltage is applied to the liquid crystal molecules. Thereafter, ultraviolet light rays are irradiated to only a display region to make liquid crystal molecules a little bit tilted with respect to the surface of a substrate, so that liquid crystal molecules are in a splay alignment state when no voltage is applied to the liquid crystal molecules. Thus, the hybrid alignment film serves as nucleus generating means for making a transition from liquid crystal molecules in a certain alignment state located outside a pixel region to the molecules in a bend alignment state.
Also, according to JP 3074640 B, the following method is described. That is, a power-on reset signal is supplied from a system such as a personal computer to a scan signal electrode to generate a strong electric field between the scan signal electrode and a common electrode. Simultaneously, a voltage equal to or larger than the critical voltage Vc required for making liquid crystal molecules stay in a bend alignment state is applied between a pixel electrode and the common electrode to make a transition from liquid crystal molecules in a splay alignment state to the same in a bend alignment state in a short time. In addition, during operation for displaying an image, operation similar to the above-mentioned operation is performed at predetermined intervals to make liquid crystal molecules stay in a bend alignment state.
According to JP 2000-321588 A, the following method is described. That is, a high voltage is applied to a common electrode in a situation where a space between pixel electrodes is made narrow, so that a strong electric field is generated not only between the common electrode and the pixel electrode, but also between a scan signal electrode located between the pixel electrodes and the common electrode and further, between a video signal electrode and the common electrode, securely making a transition from liquid crystal molecules in a splay alignment state to the same in a bend alignment state all over a display surface.
However, problems are found in the above methods. The method described in JP 09-218411 A requires a large number of spacers to uniformly be sprayed to make liquid crystal molecules stably stay in a bend alignment state. However, liquid crystal molecules around the spacer as nucleus generating means are aligned in a distorted direction, causing a leakage of light rays in a black display state.
According to the method described in JP 10-142638 A, since liquid crystal molecules near the surface of a spacer as nucleus generating means are aligned vertical to the surface thereof, liquid crystal molecules located on side surfaces of the spacer are aligned parallel to the substrate, thereby further enhancing a leakage of light rays. Furthermore, since it is necessary to spray a spacer as nucleus generating means having a diameter smaller than that of a gap between two substrates in addition to a gap material, it becomes difficult to fix within the gap the spacer having a diameter smaller than that of the gap to stably display an image.
According to the method described in JP 10-020284 A, in addition to a problem found in that liquid crystal molecules around the spacer as nucleus generating means are aligned in a distorted direction, causing a leakage of light rays in a black display state, problems arise in that the number of steps for forming the nucleus generating means increases and controlling the profile of the convex portion made of a conductive material and having a taper is difficult.
According to the method described in JP 2000-330141 A, a problem arises in that uniformly and stably making liquid crystal molecules tilted largely with respect to the surface of a substrate by using the hybrid alignment film is difficult.
The method described in JP 3074640 B is effective as means for making an initial transition from liquid crystal molecules in a certain alignment state to the same in a desired alignment state and in addition, serves to making liquid crystal molecules stably stay in a bend alignment state during operation for displaying an image by performing reset operation at predetermined time intervals even during operation for displaying an image. However, writing data to display an image in a black display state while stopping normal operation for displaying an image substantially causes deterioration in transmittance of light rays incident on a liquid crystal display device.
Although the method described in JP 2000-321588 A is effective as means for making an initial transition from liquid crystal molecules in a certain alignment state to the same in a desired alignment state, since a high voltage is applied to the common electrode, liquid crystal molecules cannot stably stay in a bend alignment state during operation for displaying an image.